yup 12-13 mpg with 35's and 4.56 gears here... I'm looking to improve that with more realtime information hopefully from torque and my android phone feeding me computer info from the OBD computer once i get everything..

 

Not just gas mileage with re-gearing but youll save your transmission specially if its automatic from tearing itself to peices with huge tires and stock gears

 

the rear o2's absolutely have an effect on fuel curve.. I'll go off on a tangent about it if provoked.. I learned this over the last several weeks..

there is a way around it, though, that will eliminate the code, AND keep your truck from guzzling..

instead of using a non-fouler to eliminate codes and trick the computer into thinking 'all is well', pull the o2, plug the hole, and leave it attached to the harness.. I used a metal hose clamp to attach it to the exhaust pipe whilst I did my testing, with the sensor hiding behind the pipe in effort to keep it out of direct air stream while driving..

the sensor can be fooled from either side.. either internally (non-fouler) or externally by blocking the little holes nearest the end of the sensor, and where the wire exits.. you accomplish the same thing with both tacts.. either way you're providing a consistent reading across the 'wire' that indicates the air inside the exhaust is comparable (with parameters) with the air on the outside..

when your ohms spike, fuel is dumped.. a lot of fuel is dumped.. when you run flat, your engine is apt to drop back to open loop at the slightest provocation.. if you can hover right across the bend, your engine thinks all is well with the sensor and the trim.. it's called cross-counting.. when the air/fuel is perfect, it sits at 0.. but it's never perfect.. so it dips a little below, or a little above.. when the 'crossing of 0' happens, the engine knows it's close to properly trimmed.. when you have a high number of 'crosses', the engine thinks it's doing a good job.. this is why you want to see a shizen load of cross counts..

when there isn't a lot of cross-counts, and it is steadily below expected parameters, it pulls fuel.. the engine dumps fuel when the voltage/ohms are high.. the difference is this: there seems to be more PCM concern with running lean than with running rich.. it will fatten you up quick like, but it's hesitant to pull fuel..

a bad, fouled, or aging sensor will gum up with oil, grease, anti-freeze (from a leak) or road gunk.. it will make the exterior of the sensor more insulated, which will make teh exterior 'samples' look more like the 'interior' samples.. and it will hold a really consistent voltage.. if the exterior is more gummed up than the interior, it's going to think it's starving for fuel (it will remain above the cross).. you'll dump a ton of fuel.. your economy will go out the window..

okay, I didn't mean to floggin' spill this all over the page, but I did.. here is the important part:

keep the sensors exterior clean.. anti-freeze will kill it, and grime, oil, grease, ect.. and it's toast.. also, the connectors should be cleaned and inspected for integrity.. there is just a little bit of voltage on that third wire.. anything providing extra resistance will limit cross counts.. limit your cross counts? watch out for lots of gas coming your way..

 

So if I gut my cat should I do the non fouler trick or should I let it hang outside of the exhaust. I am replacing the sensor though

 

Ok....I have to ask.

Are you driving like...



to get that mpg?

 

Non fouler trick.

 

both achieve the same result, but it is a heckuva lot more consistent by using the non-fouler it looks like..

I get 12~13 city.. around 14~15 highway with my rig.. It's 35's on 4.56:1's too.. I usually drive slightly quicker than traffic around me.. If I had the OE 3.92's, I would expect the city economy to drop to around ~10ish, but highway to be up around 16~17 consistently..

If I lost the heavy tires (which I will after these are spent), I bet I could hit 18ish on the highway as she sits now with nothing but the tires changed.. the weight of those things can't be understated.. they weigh 180#, which means 720 felt pounds being rotational weight.. OE was around 80#.. that's 320# rotational weight..

 

not at all.... 4.56 gears bring the load and rpms back to stock levels with 35" tires.... I have 147,000 miles on it with no real mods except shorty headers and CAI 180 degree stat and dual exhaust from the muffler back. I found a hole in the pipe right before my dowward sensor so I wonder if thats throwing my computers reading off.
I'm not stupid though...If I want to drive fast I use the right car for the job... my vette This is a truck I use it for what its good at..
of course my transmission decided to quit shifting again today so I'm not to happy with it right now..

 

ok, after I replaced the rear o2 sensor, and have run two tanks through so far. One tank was roughly 200 miles, and about 170 of that was highway/interstate driving, I averaged a hair over 11 mpg. Next tank was about 200 miles and 60 of that was highway driving, averaged 9.5. Not the improvement I was hoping for.

I did get a P1740 error on the second tank. I had this before, and replaced the tcc & overdrive solenoids, and I haven't had it since then (maybe a year ago - until now, but I rarely get on the highway). I did notice that it was bumping up and down out of overdrive a lot when I was right about 50mph, but it got better when the truck was warmed up.

Should I be looking at the driveline for losses instead of the engine?

 

The dropping in and out of O/D, (which is actually prolly TCC lockup) is usually TPS related. Test it, and see what the results are. This WILL impact fuel economy.

How old is the front O2? How's your cat? (the one on the truck......) Plugs? Wires? Cap? Rotor?